Affinage

TTC8

Tetratricopeptide repeat protein 8 · UniProt Q8TAM2

Length
541 aa
Mass
61.5 kDa
Annotated
2026-06-10
24 papers in source corpus 12 papers cited in narrative 12 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 6/6 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

TTC8/BBS8 is a ciliary protein that localizes at the base of cilia and moves bidirectionally along the axoneme, where it acts selectively in the assembly and motility of intraflagellar transport (IFT) particles and is required for ciliary protein localization and cilium maintenance across diverse cell types including sensory neurons and photoreceptors (PMID:15231740, PMID:21646512, PMID:29126234). It functions as a BBSome-associated component, and its loss destabilizes other BBSome subunits and disorganizes photoreceptor outer segments (PMID:29126234). BBS8 directly interacts with the core planar cell polarity protein Vangl2 to control asymmetric Vangl2 accumulation, stereociliary bundle orientation, and left-right asymmetry, linking it to cytoskeletal organization at both actin and microtubule structures (PMID:25605782, PMID:20643117). It also interacts with HDAC2, and BBS8 loss elevates HDAC2 to reduce K40 acetylation of ciliary α-tubulin and compromise ciliary stability, a defect reversible by HDAC inhibition (PMID:40667253). A photoreceptor-specific alternative exon (exon 2A), whose inclusion is governed by redundant intronic splicing enhancers bound by Musashi proteins, is essential for BBS8 function in photoreceptors; mutations disrupting its splicing cause nonsyndromic retinitis pigmentosa confined to the retina [PMID:20451172, PMID:25776555, PMID:bio_10.1101_2025.11.26.690869]. Beyond ciliary roles, BBS8 loss drives an EMT-like phenotype in retinal pigment epithelium and induces ectopic ciliary Hedgehog signaling in adipocyte precursors that biases them toward a fibrogenic rather than adipogenic fate (PMID:33681195, PMID:40836034).

Mechanistic history

Synthesis pass · year-by-year structured walk · 12 steps
  1. 2004 High

    Established that BBS8 is not merely ciliary but functionally embedded in the IFT machinery, defining its core molecular role.

    Evidence Loss-of-function mutations in C. elegans bbs-8 with fluorescence localization and IFT motility assays

    PMID:15231740

    Open questions at the time
    • Does not define which IFT subcomplex BBS8 assembles
    • No biochemical reconstitution of BBS8 within a complex
    • Mechanism of bidirectional movement unresolved
  2. 2010 Medium

    Showed that a retina-specific BBS8 alternative exon is the dominant photoreceptor isoform and its loss causes tissue-restricted disease, revealing tissue-specific function encoded by splicing.

    Evidence Linkage mapping, candidate sequencing, and RT-PCR of exon 2A in human nonsyndromic retinitis pigmentosa

    PMID:20451172

    Open questions at the time
    • What the 10 extra amino acids do biochemically is unknown
    • Splicing regulators not yet identified at this stage
    • Single family/lab
  3. 2010 Medium

    Connected BBS8 to planar cell polarity and left-right asymmetry, expanding its role from IFT to PCP and basal body positioning.

    Evidence Zebrafish morpholino knockdown and genetic interaction of bbs8 and vangl2 with cilia and actin imaging

    PMID:20643117

    Open questions at the time
    • Morpholino-based, no genetic mutant
    • Direct physical interaction not shown here
    • Mechanism of centrosome migration defect unclear
  4. 2011 High

    Demonstrated BBS8 is required for ciliogenesis, ciliary protein trafficking, and neuronal axon targeting in olfactory neurons, showing broad sensory dependence on BBS8.

    Evidence Bbs8-null mouse with reporter knock-in, IHC, live imaging, and electroolfactogram

    PMID:21646512

    Open questions at the time
    • Link between ciliary defect and axon mistargeting mechanistically unresolved
    • Trafficking substrates not enumerated
  5. 2015 High

    Resolved the molecular basis of retina-restricted BBS8 disease: photoreceptor-specific exon 2A inclusion makes only photoreceptors vulnerable to a splice mutation, governed by redundant intronic enhancers.

    Evidence RT-PCR across cell types, minigene splicing reporters, and intronic enhancer analysis

    PMID:25776555

    Open questions at the time
    • Identity of trans-acting splicing factors not determined here
    • Functional consequence of the encoded peptide not defined
  6. 2015 Medium

    Provided direct biochemical evidence that BBS8 binds Vangl2 and positions BBS8 upstream of core PCP asymmetry, with localization to both actin and microtubules.

    Evidence Reciprocal Co-IP, Bbs8 conditional knockout mouse, and cochlear PCP marker immunofluorescence

    PMID:25605782

    Open questions at the time
    • Whether interaction is direct or complex-mediated not fully resolved
    • Single lab
    • Mechanism linking BBS8 to Vangl2 asymmetry unclear
  7. 2018 High

    Showed BBS8 loss destabilizes the BBSome and disrupts photoreceptor outer segment architecture and ciliary microtubule acetylation, linking BBSome integrity to outer segment elaboration.

    Evidence Rod/cone-specific Bbs8 conditional knockout with Western blot of BBSome subunits, IF, ERG, and histology

    PMID:29126234

    Open questions at the time
    • Mechanism of differential subunit stabilization unknown
    • Cause of syntaxin3 mislocalization unresolved
  8. 2021 Medium

    Revealed a non-ciliary BBS8 role in RPE homeostasis, where loss produces EMT-like changes in polarity and adhesion.

    Evidence Bbs8-null mouse RPE proteomics/transcriptomics with cytoskeletal IF and polarity assays

    PMID:33681195

    Open questions at the time
    • Whether EMT phenotype is cilium-dependent unclear
    • No direct molecular effector identified
    • Single lab
  9. 2022 Medium

    Defined an additional disease mechanism whereby an exon 13 donor-site mutation causes cryptic splicing and premature termination, broadening the spectrum of TTC8 splicing pathology.

    Evidence Patient cDNA RT-PCR/Sanger sequencing and bioinformatic splice prediction

    PMID:35939099

    Open questions at the time
    • Single method, single lab
    • Protein-level consequence not directly measured
    • SC35 disruption inferred bioinformatically
  10. 2025 Medium

    Linked BBS8 loss to ectopic ciliary Hedgehog signaling that redirects adipocyte precursors toward fibrogenesis, connecting ciliary dysfunction to a tissue fate switch.

    Evidence Bbs8-/- mouse scRNA-seq trajectory analysis, flow cytometry, Hedgehog reporters, and in vitro differentiation

    PMID:40836034

    Open questions at the time
    • Direct molecular link between BBS8 and Hedgehog activation unresolved
    • Whether fibrogenesis is reversible untested
    • Single lab
  11. 2025 Medium

    Identified Musashi proteins as the trans-acting regulators that drive photoreceptor-specific Ttc8 exon inclusion, closing the loop on the splicing mechanism inferred earlier.

    Evidence Graded Msi1/Msi2 knockout allele series with RT-PCR splicing readout and ERG (preprint)

    PMID:bio_10.1101_2025.11.26.690869

    Open questions at the time
    • Preprint, not peer-reviewed
    • Direct Musashi binding to Ttc8 introns shown by sequence proximity not in vivo footprinting
    • Other regulators not excluded
  12. 2025 Medium

    Identified HDAC2 as a BBS8 partner whose deregulation explains reduced ciliary tubulin acetylation, offering a pharmacologically reversible mechanism of ciliary destabilization.

    Evidence Bbs8-/- mouse kidney phenotyping, Co-IP, acetylated-tubulin IF, and HDAC-inhibitor rescue in patient cells (preprint)

    PMID:40667253

    Open questions at the time
    • Preprint, not peer-reviewed
    • Whether interaction is direct or via BBSome unclear
    • Mechanism of HDAC2 upregulation upon BBS8 loss unknown

Open questions

Synthesis pass · forward-looking unresolved questions
  • How BBS8 mechanistically couples its BBSome/IFT role to its diverse downstream effects (Vangl2-PCP asymmetry, HDAC2-tubulin acetylation, Hedgehog activation) remains unresolved.
  • No unified structural model placing BBS8 within the BBSome
  • Causal hierarchy among ciliary, PCP, and signaling defects undefined
  • Function of the photoreceptor-specific peptide unknown

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0060090 molecular adaptor activity 2 GO:0008092 cytoskeletal protein binding 1
Localization
GO:0005929 cilium 4 GO:0005815 microtubule organizing center 2 GO:0005856 cytoskeleton 1
Pathway
R-HSA-1266738 Developmental Biology 3 R-HSA-162582 Signal Transduction 3 R-HSA-1852241 Organelle biogenesis and maintenance 3
Partners
HDAC2VANGL2
Complex memberships
BBSome

Evidence

Reading pass · 12 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2004 C. elegans BBS-8 protein localizes predominantly at the base of cilia and moves bidirectionally along the ciliary axoneme, similar to IFT proteins. BBS-8 is required for the normal localization and motility of IFT proteins OSM-5/Polaris and CHE-11 (and to a lesser extent CHE-2), demonstrating that BBS-8 plays a selective role in the assembly and/or function of IFT particle components. Loss-of-function mutations in C. elegans bbs-8; fluorescence microscopy of protein localization; functional IFT motility assays Genes & development High 15231740
2010 A retina-specific alternative exon of BBS8 (exon 2A) encodes 10 amino acids and represents the major BBS8 mRNA isoform in mammalian photoreceptors. A splice-site mutation eliminating this exon causes nonsyndromic retinitis pigmentosa restricted to the retina, establishing that this photoreceptor-specific sequence is pivotal for BBS8 function in the retina. Genome-wide linkage mapping; candidate gene sequencing; RT-PCR and expression analysis of retina-specific exon; segregation analysis American journal of human genetics Medium 20451172
2011 BBS8 localizes in olfactory sensory neurons (OSNs) at the dendritic knob in a shell-like structure surrounding basal bodies. Loss of BBS8 causes near-complete loss of cilia from OSNs, mislocalization of ciliary-enriched proteins, and aberrant axon targeting of OSNs to the olfactory bulb, demonstrating BBS8 is required for ciliary protein localization and axon targeting in OSNs. Bbs8-null mouse with fluorescent reporter knock-in; immunohistochemistry; ex vivo live imaging of SLP3-eGFP knock-in; electroolfactogram Proceedings of the National Academy of Sciences of the United States of America High 21646512
2015 BBS8 directly interacts with the core planar cell polarity (PCP) molecule Vangl2, as demonstrated by co-immunoprecipitation. Loss of Bbs8 disrupts asymmetric accumulation of Vangl2 in cochlear cells and causes mis-oriented hair cell stereociliary bundles, placing BBS8 upstream of core PCP asymmetry. BBS8 also localizes to filamentous actin as well as microtubules. Co-immunoprecipitation (BBS8–Vangl2); Bbs8 conditional knockout mouse; immunofluorescence of cochlear PCP markers; subcellular fractionation/localization Development (Cambridge, England) Medium 25605782
2010 Bbs8 and the core PCP protein Vangl2 interact and act synergistically in zebrafish to establish left-right asymmetry; loss of bbs8 disrupts cilia at Kupffer's vesicle, causes defective actin organization, and leads to nuclear mislocalisation implying defective centrosome/basal body migration and apical docking. Zebrafish morpholino knockdown of bbs8 and vangl2; genetic interaction/epistasis; cilia imaging; actin staining Developmental biology Medium 20643117
2015 Cell-type-specific alternative splicing of BBS8 exon 2A (the IVS1-2A>G mutation) causes photoreceptor-specific loss of BBS8 protein because: (1) in photoreceptors, exon 2A is included and the mutation causes a frameshift eliminating the protein; (2) in all other cell types, exon 2A is skipped, rendering those cells immune to the mutation. Splicing of exon 2A in photoreceptors is directed exclusively by redundant splicing enhancers in the flanking introns. RT-PCR in multiple cell types; minigene splicing reporters; analysis of intronic splicing enhancer sequences; patient and control cell RNA analysis Molecular and cellular biology High 25776555
2018 BBS8 is required for normal elaboration of photoreceptor outer segments. Loss of BBS8 leads to: concomitant decrease in BBSome subunits BBS2 and BBS5 (and increase in BBS1 and BBS4); disorganized and shortened outer segments; altered distribution of photoreceptor axonemal proteins; hyper-acetylation of ciliary microtubules; and mislocalization of syntaxin3 to the outer segment (a protein normally absent from OS). Bbs8 conditional knockout mouse (rod- and cone-specific); Western blot for BBSome subunit levels; immunofluorescence; ERG; histology Human molecular genetics High 29126234
2021 Loss of Bbs8 in the retinal pigment epithelium (RPE) causes changes in cytoskeletal and cell adhesion molecules, defective cellular polarization and morphology, and an epithelial-to-mesenchymal transition (EMT)-like phenotype, demonstrating a role for BBS8 in RPE homeostasis beyond ciliary function. Bbs8-null mouse RPE; proteomics and transcriptomics (combinatorial omics); immunofluorescence for cytoskeletal markers; cell polarity assays Frontiers in cell and developmental biology Medium 33681195
2022 A missense mutation c.1347G>C at the last base of exon 13 of TTC8 disrupts the canonical donor splice site, activating a cryptic splice site 77 bases into intron 13, causing intron retention, a frameshift, and a premature termination codon in exon 14. The mutation also disrupts the binding site for SC35 splicing factor and causes duplication and fusion of exon 15. RT-PCR and Sanger sequencing of patient cDNA; bioinformatic splice site prediction (SpliceAid2); minigene/functional RNA analysis Molecular genetics and genomics : MGG Medium 35939099
2025 Loss of BBS8 in adipocyte precursor cells (APCs) induces ectopic ciliary Hedgehog signaling, driving a phenotypic switch from the stem-cell-like P1 APC subpopulation to a fibrogenic progenitor state (characterized by ECM remodeling and upregulation of CD9), bypassing the committed P2 progenitor state and altering adipogenesis. Bbs8-/- mouse model; single-cell RNA sequencing; flow cytometry; Hedgehog pathway reporter assays; in vitro differentiation assays The EMBO journal Medium 40836034
2025 BBS8 physically interacts with HDAC2. Loss of BBS8 upregulates HDAC2 in kidneys, mouse embryonic fibroblasts, and patient-derived urine renal epithelial cells, leading to reduced K40 acetylation of α-tubulin within primary cilia and compromised ciliary stability. Pharmacological HDAC2 inhibition restores tubulin acetylation in BBS8-deficient cells. Bbs8-/- mouse kidney phenotyping; proteomics; co-immunoprecipitation (BBS8–HDAC2); immunofluorescence for acetylated tubulin; pharmacological rescue with HDAC inhibitor in patient-derived cells bioRxivpreprint Medium 40667253
2025 Musashi proteins (MSI1/MSI2) promote inclusion of the photoreceptor-specific alternative exon of Ttc8 by binding to proximal downstream intronic sequences. A single Musashi allele (from either Msi1 or Msi2) is sufficient to maintain high inclusion levels of the Ttc8 photoreceptor-specific exon and photoreceptor function. Combined Msi1/Msi2 knockout mice with graded allele reduction; RT-PCR splicing analysis of Ttc8 photoreceptor-specific exon; photoreceptor function assays (ERG) bioRxivpreprint Medium bio_10.1101_2025.11.26.690869

Source papers

Stage 0 corpus · 24 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2004 Loss of C. elegans BBS-7 and BBS-8 protein function results in cilia defects and compromised intraflagellar transport. Genes & development 285 15231740
2010 A splice-site mutation in a retina-specific exon of BBS8 causes nonsyndromic retinitis pigmentosa. American journal of human genetics 99 20451172
1985 Effect of RP51 gene dosage alterations on ribosome synthesis in Saccharomyces cerevisiae. Molecular and cellular biology 92 3915776
2011 Loss of Bardet-Biedl syndrome protein-8 (BBS8) perturbs olfactory function, protein localization, and axon targeting. Proceedings of the National Academy of Sciences of the United States of America 85 21646512
2015 Ciliary proteins Bbs8 and Ift20 promote planar cell polarity in the cochlea. Development (Cambridge, England) 61 25605782
2010 Bbs8, together with the planar cell polarity protein Vangl2, is required to establish left-right asymmetry in zebrafish. Developmental biology 61 20643117
2018 Bardet-Biedl syndrome-8 (BBS8) protein is crucial for the development of outer segments in photoreceptor neurons. Human molecular genetics 54 29126234
2015 Alternative Splicing Shapes the Phenotype of a Mutation in BBS8 To Cause Nonsyndromic Retinitis Pigmentosa. Molecular and cellular biology 37 25776555
2009 BBS7 and TTC8 (BBS8) mutations play a minor role in the mutational load of Bardet-Biedl syndrome in a multiethnic population. Human mutation 34 19402160
2014 A novel mutation in TTC8 is associated with progressive retinal atrophy in the golden retriever. Canine genetics and epidemiology 29 26401321
2005 BBS8 is rarely mutated in a cohort of 128 Bardet-Biedl syndrome families. Journal of human genetics 25 16308660
2015 Confirmation of TTC8 as a disease gene for nonsyndromic autosomal recessive retinitis pigmentosa (RP51). Clinical genetics 23 26195043
2021 Loss of Ciliary Gene Bbs8 Results in Physiological Defects in the Retinal Pigment Epithelium. Frontiers in cell and developmental biology 17 33681195
2019 A novel compound heterozygous mutation in TTC8 identified in a Japanese patient. Human genome variation 11 30886724
2020 Deletion in the Bardet-Biedl Syndrome Gene TTC8 Results in a Syndromic Retinal Degeneration in Dogs. Genes 8 32962042
2022 A missense mutation in TTC8/BBS8 affecting mRNA splicing in patients with non-syndromic retinitis pigmentosa. Molecular genetics and genomics : MGG 7 35939099
2016 Alternative Isoform Analysis of Ttc8 Expression in the Rat Pineal Gland Using a Multi-Platform Sequencing Approach Reveals Neural Regulation. PloS one 7 27684375
2022 Behavioral Phenotyping of Bbs6 and Bbs8 Knockout Mice Reveals Major Alterations in Communication and Anxiety. International journal of molecular sciences 6 36498834
2025 BBS8-dependent ciliary Hedgehog signaling governs cell fate in the white adipose tissue. The EMBO journal 4 40836034
2022 Allele frequency of SLC4A3 (PRA1), TTC8 (PRA2), and PRA-prcd mutations in golden retrievers in Brazil. Frontiers in veterinary science 2 36325094
2019 Targeted sequencing of linkage region in Dominican families implicates PRIMA1 and the SPATA7-PTPN21-ZC3H14-EML5-TTC8 locus in carotid-intima media thickness and atherosclerotic events. Scientific reports 1 31406157
2016 FAM161A and TTC8 are Differentially Expressed in Non-Allelelic Early Onset Retinal Degeneration. Advances in experimental medicine and biology 1 26427412
2026 A novel protein truncating mutation of TTC8 causes Bardet-Biedl Syndrome (BBS) in a Pakistani family. Genetics and molecular biology 0 41686921
2025 Loss of Bbs8 leads to cystic kidney disease in mice with reduced acetylation of ciliary alpha-tubulin through HDAC2. bioRxiv : the preprint server for biology 0 40667253

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